In the drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The rapid advance toward finer pattern rules is grounded on the development of a projection lens with an increased NA, a resist material with improved performance, and exposure light of a shorter wavelength. In particular, the change-over from i-line (365 nm) to shorter wavelength KrF excimer laser (248 nm) brought about a significant innovation, enabling mass-scale production of 0.18 micron rule devices. To the demand for a resist material with a higher resolution and sensitivity, acid-catalyzed chemical amplification positive working resist materials are effective as disclosed in U.S. Pat. No. 4,491,628 and U.S. Pat. No. 5,310,619 (JP-B 2-27660 and JP-A 63-27829). They now become predominant resist materials especially adapted for deep UV lithography.
Resist materials adapted for KrF excimer lasers enjoyed early use on the 0.3 micron process, went through the 0.25 micron rule, and currently entered the mass production phase on the 0.18 micron rule. Engineers have started investigation on the 0.15 micron rule, with the trend toward a finer pattern rule being accelerated. A wavelength change-over from KrF to shorter wavelength ArF excimer laser (193 nm) is expected to enable miniaturization of the design rule to 0.13 μm or less. Since conventionally used novolac resins and polyvinylphenol resins have very strong absorption in proximity to 193 nm, they cannot be used as the base resin for resists. To ensure transparency and dry etching resistance, some engineers investigated acrylic and alicyclic (typically cycloolefin) resins as disclosed in JP-A 9-73173, JP-A 10-10739, JP-A 9-230595 and WO 97/33198.
With respect to F2 laser light (157 nm) which is expected to enable further miniaturization to 0.10 μm or less, more difficulty arises in insuring transparency because it was found that acrylic resins are not transmissive to light at all and those cycloolefin resins having carbonyl bonds have strong absorption. Polymers having benzene rings have a somewhat improved transmittance in proximity to 160 nm wavelength, which is far below the practically acceptable level. It was found that in single layer resists, reducing carbon-to-carbon double bonds as typified by benzene rings and carbon-to-carbon double bonds as typified by benzene rings and carbon-to-oxygen double bonds as typified by carbonyl groups is essential for insuring a light transmittance (see International Work Shop 157 nm Lithography MIT-LL, Boston, Mass., May 5, 1999). It was reported in J. Vac. Sci. Technol., B17(6), November/December 1999 that introduction of fluorine is effective to improve transmittance. A number of fluorinated polymers for resist compositions were proposed in J. Photopolymer Sci. and Technol., Vol. 13, No. 4 (2000), pp. 657-664 and pp. 451-458. However, the transmittance of these polymers does not reach that of polyhydroxystyrene and derivatives thereof adapted for KrF exposure and poly(meth)acrylic derivatives and polycycloolefin derivatives adapted for ArF exposure.
As is known in the art, the two-layer resist method is advantageous in forming a high-aspect ratio pattern on a stepped substrate. It is also known that in order that a two-layer resist film be developable with a common alkaline developer, high molecular weight silicone compounds having hydrophilic groups such as hydroxyl and carboxyl groups must be used.
Among silicone base chemically amplified positive resist compositions, recently proposed were those compositions for KrF exposure comprising a base resin in the form of polyhydroxybenzylsilsesquioxane, which is a stable alkali-soluble silicone polymer, in which some phenolic hydroxyl groups are blocked with t-BOC groups, in combination with a photoacid generator (see SPIE vol. 1925 (1993), p. 377). Positive resist compositions comprising as a base a silsesquioxane of the type in which cyclohexyl-carboxylic acid is substituted with an acid labile group were proposed for ArF exposure (see JP-A 10-324748, JP-A 11-302382, and SPIE vol. 3333-07 (1998), p. 62). Also proposed were silicone-containing polymers prepared from silicon-containing acrylic monomers (see JP-A 9-110938, J. Photopolymer Sci. and Technol., Vol. 9, No. 3 (1996), pp. 435-446).
The silicon-containing polymer of the acrylic pendant type has the drawback that its resistance to dry etching with oxygen plasma is weak as compared with the silsesquioxane polymer. A low silicon content and a different polymer main skeleton account for this weak dry etching resistance. The siloxane pendant type polymer has the other drawback of poor developer wetting that it is likely to repel a developer solution. SPIE vol. 3678, pp. 214, 241 and 562 describes a polymer comprising recurring units of the trisilane or tetrasilane pendant type having an increased silicon content and a silicon-containing substituent which can be eliminated with acid. However, since di- and polysilane compounds exhibit strong absorption at a wavelength of less than 200 nm, an increased introduction of such silanes undesirably leads to a lower transmittance. Besides, an attempt of introducing silicon into acid labile groups is reported in SPIE vol. 3678, p. 420. Because of a low acid elimination ability, there are drawbacks including low environmental stability and a T-top profile.
The applicant/assignee proposed in U.S. Ser. No. 09/726,592 or Japanese Patent Application No. 11-342380, an acid labile group in the form of a cyclic hydrocarbon group having silicon introduced therein. This polymer has the advantages of an improved acid elimination ability and prevention of T-top profiling and allows dry etching resistance to be enhanced by introducing two or more silicon atoms in a single cyclic hydrocarbon group. The presence of a carbon atom between silicon atoms shirks formation of a disilane bond and eliminates a concern about a loss of transmittance to ArF exposure.
However, the most serious drawback of acrylic pendant type silicon-containing polymers is a low glass transition temperature (Tg). Since chemically amplified resist compositions are designed such that elimination reaction of acid labile groups is induced by post-exposure baking (PEB), the polymer should have a Tg equal to or above the baking temperature. The PEB temperature is usually 80 to 150° C., and a Tg of about 150° C. is necessary. If PEB is performed above the Tg of the polymer, the acid diffuses through the thermally flowing polymer, losing control on pattern dimensions after development.
Another drawback of acrylic pendant type silicon-containing polymers is that when observed under a dimension measurement SEM, line dimensions are reduced during measurement. This is because ester groups are cleaved by irradiation of electron beams and the resulting ester end moieties volatilize off. A further drawback is slimming of a resist film upon exposure to a F2 laser. This is also because volume shrinkage occurs as a result of ester groups being cleaved by irradiation of VUV laser beams. When a polymer having silicone-pendant ester groups is irradiated with VUV laser beams, silicone compounds can volatilize out of the polymer. The volatilized silicone compounds will deposit on the surface of a projection lens, leading to a transmittance decline. It is reported that deposits of silicone compounds are difficult to remove as compared with hydrocarbon deposits.